Resonant accelerometer

Abstract

An accelerometer is provided by a sample mass suspended in a central area of a support by pairs of resonating arms. One pair of arms lies on one axis through the sample mass. Another pair of arms lies on a second axis through the sample mass perpendicular to the one axis. Acceleration of the mass and support is detected by a measured change in resonant frequency of the arms of a pair. The measured change in resonant frequency is the magnitude of the acceleration and the axis along which the pair of arms lies provides the direction of the acceleration. Orthogonal components of acceleration are simultaneously measured by the pairs of arms lying on perpendicular axes. Electrostatic force-rebalance techniques and other known techniques for measuring acceleration in a direction perpendicular to the axes of the pairs of arms are readily incorporated to provide a third direction measurement of acceleration. The accelerometer is fabricated in a monolithic process which employs micromachining techniques.

We claim: 
     
       1. An accelerometer comprising: a weight;   at least 2 micromachined mechanical beams for substantially equally suspending the weight from a support, one arm having one end attached to one side of the weight and the second arm having one end attached to an opposite side of the weight such that respective longitudinal axes of the one and second arms form a common axis through the weight, said arms vibrating at respective resonant frequencies in respective planes through the common axis; and   means for measuring a difference in resonant frequencies between said one arm and second arm to provide an indication of acceleration in a direction along the common axis, the respective resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support.   
     
     
       2. An accelerometer as claimed in claim 1 wherein said arms are excited to vibrate by electrostatic means. 
     
     
       3. An accelerometer as claimed in claim 2 wherein the electrostatic means include a drive electrode adjacent to each arm centrally located along the longitudinal axis of the respective arm. 
     
     
       4. An accelerometer as claimed in claim 2 wherein the electrostatic means comprise: a drive electrode for each arm, each drive electrode positioned across a gap from the respective arm;   at least one sense electrode for each arm, each sense electrode positioned across the gap from the respective arm and adjacent to the drive electrode of the respective arm, vibration of the respective arm producing a time-varying capacitance across the gap;   a feedback vibration-detection circuit for each arm connected to the drive electrode and the sense electrodes of the respective arm, the circuit sensing through the sense electrode changes in the time varying capacitance of the respective arm and thereby sensing changes in the vibration of the arm and the circuit applying voltage to the drive electrodes as a function of the sensed changes in capacitance.   
     
     
       5. A accelerometer as claimed in claim 4 wherein said sense and drive electrodes of a respective arm are positioned across the gap by an extension of the support along opposite longitudinal sides of the arm, the extension along each side supporting the sense and drive electrodes such that the sense and drive electrodes bridge over the arm between the extension on opposite longitudinal sides of the arm. 
     
     
       6. An accelerometer as claimed in claim 4 wherein said means for measuring include a frequency measuring circuit for each arm connected to the feedback vibration detection circuit of the respective arm. 
     
     
       7. An accelerometer as claimed in claim 1 wherein the arms are resonated by an electromechanical oscillator. 
     
     
       8. An accelerometer as claimed in claim 1 wherein dimensions of said weight and arms are substantially small compared to dimensions of a subject to which the support is attached. 
     
     
       9. An accelerometer as claimed in claim 8 wherein the dimensions of each arm includes a length of about 50 μm to about 500 μm;   a width of about 10 μm to about 100 μm; and   a thickness of less than about 5 μm.   
     
     
       10. An accelerometer as claimed in claim 1 further comprising a plurality of constraining arms attached to the weigh to suppress rotation of the weight. 
     
     
       11. An accelerometer as claimed in claim 1 wherein four arms suspend the weight, two arms forming a common first axis through the weight, the other two arms forming a common second axis through the weight perpendicular to the first axis, the change in difference in resonant frequencies between the two arms of the first axis providing an indication of a component of acceleration along the first axis at the same time the change in difference in resonant frequencies between the other two arms provide an indication of an orthogonal component of acceleration along the second axis. 
     
     
       12. An accelerometer as claimed in claim 11 further comprising means for measuring acceleration in a direction perpendicular to the first and second axes. 
     
     
       13. An accelerometer as claimed in claim 12 wherein the means for measuring acceleration in a direction perpendicular to the axes include electrostatic force balance means. 
     
     
       14. An accelerometer as claimed in claim 1 wherein the means for measuring include a frequency measuring circuit for each arm. 
     
     
       15. An accelerometer comprising: a weight centrally positioned within a support;   at least two pairs of micromachined mechanical beams for substantially equally suspending the weight from the support, each pair having one arm with one end attached to one side of the weight and a second arm with one end attached to an opposite side of the weight and respective opposite ends of the one and second arms attached to the support such that the one and second arms form a common longitudinal axis through the weight, the formed axes of the two pairs of arms being perpendicular to each other;   means for vibrating each arm at a respective resonant frequency in a respective plane through its respective common axis; and   means for simultaneously measuring in each pair of arms a change in difference in resonant frequencies between the one and second arms of the pair to provide an indication of acceleration in a direction along the respective common axis formed by the pair of arms, the simultaneous measuring in the two pairs providing an indication of orthogonal components of acceleration, the resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support.   
     
     
       16. An accelerometer as claimed in claim 15 wherein the the means for vibrating each arm include: a drive electrode for each arm, each drive electrode positioned across a gap from the respective arm;   a pair of sense electrodes for each arm, one sense electrode positioned adjacent to one side of the drive electrode and the other sense electrode positioned adjacent to an opposite side of the drive electrode, the pair of sense electrodes positioned across the gap from the respective arm, vibration of the respective arm producing a time varying capacitance across the gap; and   a feedback vibration-detection circuit for each arm connected to the drive electrode and pair of sense electrodes of the respective arm, the circuit sensing through the pair of sense electrodes changes in the time varying capacitance and thereby the changes in vibration of the respective arm, and the circuit causing the drive electrode to exert a force on the arm as a function of the sensed changes in vibration to maintain vibration of each arm at the respective resonant frequency.   
     
     
       17. Accelerometer as claimed in claim 16 wherein the pair of sense electrodes and drive electrode of a respective arm are positioned across the gap by an extension of the support along opposite longitudinal sides of the arm supporting end regions of the sense and drive electrodes such that the sense and drive electrodes bridge across the gap over the arm between the extension on opposite longitudinal sides of the arm. 
     
     
       18. An accelerometer as claimed in claim 16 wherein said means for simultaneously measuring includes a frequency measuring circuit for each arm connected to the feedback vibration detection circuit. 
     
     
       19. An accelerometer as claimed in claim 15 further comprising a plurality of constraining arms attached to the weight to suppress rotation of the weight. 
     
     
       20. An accelerometer as claimed in claim 15 wherein said measuring means includes a frequency measuring circuit for each arm. 
     
     
       21. An accelerometer as claimed in claim 15 further comprising means for measuring acceleration in a direction perpendicular to the axes formed by the two pairs of arms. 
     
     
       22. An accelerometer as claimed in claim 21 wherein the means for measuring acceleration in a direction perpendicular to the axes include electrostatic force balance means. 
     
     
       23. An accelerometer comprising: a weight;   at least two arms suspending the weight from a support, one arm having one end attached to one side of the weight and the second arm having one end attached to an opposite side of the weight such that respective longitudinal axes of the one and second arms form a common axis through the weight, said arms being excited by electrostatic means such that they vibrate at respective resonant frequencies; and   means for measuring a difference in resonant frequencies between said one arm and second arm to provide an indication of acceleration in a direction along the common axis, the respective resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support.   
     
     
       24. An accelerometer as claimed in claim 23 wherein the electrostatic means include a drive electrode adjacent to each arm centrally located along the longitudinal axis of the respective arm. 
     
     
       25. An accelerometer as claimed in claim 23 wherein the electrostatic means comprise: a drive electrode for each arm, each drive electrode positioned across a gap from the respective arm;   at least one sense electrode for each arm, each sense electrode positioned across the gap from the respective arm and adjacent to the drive electrode of the respective arm, vibration of the respective arm producing a time-varying capacitance across the gap;   a feedback vibration detection circuit for each arm connected to the drive electrode and the sense electrodes of the respective arm, the circuit sensing through the sense electrodes, changes in the time varying capacitance of the respective arm and thereby sensing changes in the vibration of the arm and the circuit applying voltage to the drive electrodes as a function of the sensed changes in capacitance.   
     
     
       26. An accelerometer as claimed in claim 25 wherein said sense and drive electrodes of a respective arm are positioned across the gap by an extension of the support along opposite longitudinal sides of the arm, the extension along each side supporting the sense and drive electrodes such that the sense and drive electrodes bridge over the arm between the extension on opposite longitudinal sides of the arm. 
     
     
       27. An accelerometer as claimed in claim 25 wherein said means for measuring include a frequency measuring circuit for each arm connected to the feedback vibration detection circuit of the respective arm. 
     
     
       28. An accelerometer comprising: a weight;   at least two arms for suspending the weight from a support, one arm having one end attached to one side of the weight and the second arm having one end attached to an opposite side of the weight such that respective longitudinal axes of the one and second arms form a common axis through the weight, said arms vibrating at respective resonant frequencies,   said weight and arms having dimensions substantially small compared to dimensions of a subject to which the support is attached, the dimensions of each arm including   a length of about 50 microns to about 500 microns;   a width of about 10 microns to about 100 microns; and   a thickness of less than about 5 microns; and   means for measuring a difference in resonant frequencies between said one arm and second arm to provide an indication of acceleration in a direction along the common axis, the respective resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support.   
     
     
       29. An accelerometer comprising: a weight;   at least two arms for suspending the weight from a support, one arm having one end attached to one side of the weight and the second arm having one end attached to an opposite side of the weight such that respective longitudinal axes of the one and second arms form a common axis through the weight, said arms vibrating at respective resonant frequencies; and   means for measuring a difference in resonant frequencies between said one arm and second arm to provide an indication of acceleration in a direction along the common axis, the respective resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support,   the means for measuring including a frequency measuring circuit for each arm.   
     
     
       30. An accelerometer comprising: a weight centrally positioned within a support;   at least two pairs of arms for suspending the weight from the support, each pair having one arm with one end attached to one side of the weight and a second arm with one end attached to an opposite side of the weight and respective opposite ends of the one and second arms attached to the support such that the one and second arms form a common longitudinal axis through the weight, the formed axes of the two pairs of arms being perpendicular to each other;   means for vibrating each arm at a respective resonant frequency, the means for vibrating each arm including: a drive electrode for each arm, each drive electrode positioned across a gap form the respective arm;   a pair of sense electrodes for each arm, one sense electrode positioned adjacent one side of the drive electrode and the other sense electrode positioned adjacent to an opposite side of the drive electrode, the pair of sense electrodes positioned across the gap from the respective arm, vibration of the respective arm producing a time varying capacitance across the gap; and   a feedback vibration detection circuit for each arm connected to the drive electrode and pair of sense electrodes of the respective arm, the circuit sensing through the pair of sense electrodes changes in the time varying capacitance and thereby the changes in vibration of the respective arm, and the circuit causing the drive electrode to exert a force on the arm as a function of the sensed changes in vibration to maintain vibration of each arm at the respective resonant frequency;      and   means for simultaneously measuring in each pair of arms a difference in resonant frequencies between the one and second arms of the pair to provide an indication of acceleration in a direction along the respective common axis formed by the pair of arms, the simultaneous measuring in the two pairs providing an indication of orthogonal components of acceleration, the resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support.   
     
     
       31. An accelerometer as claimed in claim 30 wherein the pair of sense electrodes and drive electrode of a respective arm are positioned across the gap by an extension of the support along opposite longitudinal sides of the arm supporting end regions of the sense and drive electrodes such that the sense and drive electrodes bridge across the gap over the arm between the extension on opposite longitudinal sides of the arm. 
     
     
       32. An accelerometer as claimed in claim 30 wherein said means for simultaneously measuring includes a frequency measuring circuit for each arm connected to the feedback vibration detection circuit. 
     
     
       33. An accelerometer comprising: a weight centrally positioned within a support;   at least two pairs of arms for suspending the weight from the support, each pair having one arm with one end attached to one side of the weight and a second arm with one end attached to an opposite side of the weight and respective opposite ends of the one and second arms attached to the support such that the one and second arms form a common longitudinal axis through the weight, the formed axes of the two pairs of arms being perpendicular to each other;   means for vibrating each arm at a respective resonant frequency; and   means for simultaneously measuring in each pair of arms a difference in resonant frequencies between the one and second arms of the pair to provide an indication of acceleration in a direction along the respective common axis formed by the pair of arms, the simultaneous measuring in the two pairs providing an indication of orthogonal components of acceleration, the resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of support, said means for measuring including a frequency measuring circuit for each arm.   
     
     
       34. An accelerometer comprising: a weight;   at least 4 arms for substantially equally suspending the weight from a support, two arms forming a common first axis through the weight, the other two arms forming a common second axis through the weight perpendicular to the first axis, a change in difference in resonant frequencies between the two arms of the first axis providing an indication of a component of acceleration along the first axis at the same time a change in difference in resonant frequencies between the other two arms provide an indication of an orthogonal component of acceleration along the second axis;   means for measuring the change in difference in resonant frequencies between the two arms of the first axis and the change in difference in resonant frequencies between the other two arms, the respective resonant frequency of each arm along a common axis changing as a function of extension or compression of the arm due to acceleration of the support in a direction along the common axis; and   means for measuring acceleration in a direction perpendicular to the first and second axes, the means including electrostatic force balance means.   
     
     
       35. An accelerometer comprising: a weight centrally positioned within a support;   at least two pairs of arms for substantially equally suspending the weight from the support, each pair having one arm with one end attached to one side of the weight and a second arm with one end attached to an opposite side of the weight and respective opposite ends of the one and second arms attached to the support such that the one and second arms form a common longitudinal axis through the weight, the formed axis of the two pairs of the arms being perpendicular to each other;   means for vibrating each arm at a respective resonant frequency in a respective plane through its respective common axis;   means for simultaneously measuring in each pair of arms a change in difference in resonant frequencies between the one and second arms of the pair to provide an indication of acceleration in a direction along the respective common axis formed by the pair of arms, the simultaneous measuring in the two pairs providing an indication of orthogonal components of acceleration, the resonant frequency of each arm changing as a function of extension or compression of the arm due to acceleration of the support; and   electrostatic force balance means for measuring acceleration in a direction perpendicular to the axes formed by the two pairs of arms.